Add active users to telemetry and LiveDashboard

[Dennis Beatty]

We should track the number of users that are currently connected, whether in a lobby or game. That number should be exposed on the LiveDashboard so that we can log in and see that number easily

See the active_users example at the bottom of this section of the telemetry docs: https://hexdocs.pm/telemetry_metrics/Telemetry.Metrics.html#module-vm-metrics